The present invention relates to a technology to perform remote copying of data between storage devices in a computer system having multiple storage devices.
In a computer system that executes important operational processes such as money deposit and withdrawal at financial institutions, in order to maintain operations even in the event of a failure, strong demand exists for improvements in fault tolerance and availability. At present, a technology called ‘remote copying’ is often used in order to improve fault tolerance and availability. Remote copying is a technology for copying data between multiple storage devices installed at different locations, such as in different sites. According to this remote copying, even if a copy source storage device fails due to a natural disaster or the like, work processing can be continued using the copy destination storage device.
There are two substantially different forms of remote copying: synchronous copying and asynchronous copying. In synchronous copying, when a data update instruction is issued from a host computer, the completion of the update operation is reported to the host computer only after data updating has been completed on both the copy source storage device and the copy destination storage device. In other words, in synchronous copying, the two sets of data are synchronized at the same moment that the data update operation is completed, excluding the time during which the data is transmitted over the communication path and the very short amount of time required for the data to be written to the copy destination storage device. Because synchronous copying requires as short a response time from the copy destination storage device as possible, it is preferred that it be carried out between two storage devices located in adjacent areas no more than 100 km apart.
Conversely, in asynchronous copying, when a data update instruction is issued from a host computer, the completion of the update operation is reported to the host computer after updating of the data on the copy source storage device has been completed, regardless of whether updating of the data on the copy destination storage device has been completed. In other words, the update completion report can be reported to the host computer from the copy source storage device before the data received from the host computer is transmitted to the copy destination storage device. Consequently, the updated data is asynchronously copied. sequentially to the copy destination storage device in accordance with a timing scheme determined by the copy source storage device. In other words, asynchronous copying cannot guarantee that the data on the copy source storage device and the copy destination storage device will be identical at any given point in time, but it does offer the advantage of releasing the host computer from the data update operation quickly. Therefore, asynchronous copying can be used without any particular restriction on the distance between the two storage devices.
Disclosures of this type of remote copying include, for example, Japanese Patent Laid-Open No. 2003-122509, which discloses a remote copying technology wherein synchronous copying is carried out from a first storage subsystem to a nearby second storage subsystem, and asynchronous copying is carried out from the first storage subsystem to a third storage subsystem which is located far away from the first storage subsystem. Japanese Patent Laid-Open No. 2003-305856 discloses a technology whereby data is copied via asynchronous copying between two data centers.